The present invention relates to a signal transmission method used in a base station of a radio system, for example, the base station being composed of a motherboard structure and printed circuit board units transmitting signals to one another.
In prior art motherboard structures, units coupled to the motherboard structure communicate with one another by signals transmitted and received by the units. Generally, extremely high-frequency signals, such as clock signals,-which enable, for example, different units to be synchronized with one another, are transmitted through a motherboard structure. In addition, the units transmit RF signals to other units through a motherboard. Frequencies of high-frequency clock signals are reduced before they are transmitted to the motherboard, after which the clock frequency is again raised to the original value. However, changing the clock frequency to and fro may have caused frequency changes to the clock signal frequency.
Since a large number of different signals is transmitted through the motherboard, much wiring is needed thereon. However, space on the motherboard is restricted, for which reason designing the motherboard has become problematic. This inevitably results in dense wiring, and therefore signals transmitted along the wiring interfere, at least to some extent, with one another.
The aim has been to alleviate the above problems by conducting interfering signals to separate cables by which the motherboard is passed. However, in practice it is difficult and relatively expensive to use the cables. If separate cables have to be used frequently, signals to be transmitted along the cables also generate different types of interference. Furthermore, transmission capacity of long cables is not adequate enough for all applications. It has also been problematic to use the cables because of faulty couplings made at the installation stage of the cables and cracking of the cable insulating material at low temperatures.
An object of the invention is thus to provide a method and a motherboard structure to solve the above problems. This is achieved by a method of the type presented in the introduction, which is characterized by when the printed circuit board units of the base station communicate with one another by signals which are transmitted through the common motherboard structure, of these signals converting into optical signals those with a low power and a high frequency, such as clock signals and RF signals and conducting each optical signal into an optical fibre of its own, each optical fibre being placed inside a fibre sheet attached to the motherboard structure, converting the optical signals that have propagated in the fibre sheet back into electrical signals and conducting each signal that has been converted into an electrical signal to the printed circuit board unit.
The invention also relates to a motherboard structure used in a base station of a radio system, for example, and comprises printed circuit board units communicating with the motherboard structure, the printed circuit board units communicating with one another by signals.
The motherboard structure of the invention is characterized in that the motherboard structure comprises an optical fibre sheet with optical fibres arranged therein, first converter means receiving from the printed circuit board units electrical low-power and high-frequency signals, such as clock signals and RF signals, and converting the received signals into optical signals, each optical signal being conducted into an optical fibre of its own in the fibre sheet, and second converter means converting the optical signals that have propagated in the fibre sheet back into electrical signals which are conducted to the printed circuit board units after the conversion.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea of using a fibre sheet in the motherboard structure of a base station and transmitting in the fibre sheet signals between the units arranged in the base station, which makes it possible to prevent the signals transmitted through the motherboard from causing interference on the motherboard.
The method and motherboard structure of the invention provide many advantages. An optical fibre sheet arranged in the motherboard enables the generation of electromagnetic interference, for example, to be reduced on the motherboard. When signals which probably generate interference are transmitted on the fibre sheet, the rest of the motherboard structure can be simplified. The use of the fibre sheet enables the layers of the printed circuit board to be reduced, which makes it is easier and faster to design the motherboard. Furthermore, thanks to fewer layers on the printed circuit board, the manufacturing costs of the motherboard become considerably smaller.
When the fibre sheet is used, different types of cables are not required, and therefore the risk of faulty couplings on the motherboard is eliminated. The use of the fibre sheet enables the size of the motherboard to be reduced. Furthermore, the fibre sheet is suitable for use in a wide range of temperatures. Since the structure of the fibre sheet differs widely from that of the cables, for example, the insulation material no longer becomes cracked at low temperatures. The use of a single fibre or a fibre sheet is also much more cost-effective than the use of the cables.